1. Field of the Invention
The present invention relates to an optical modulator and, more particularly, to a Mach-Zehnder type optical modulator which allows the interaction appropriately between signal electrodes and an optical waveguide, only by matching phases at input ends of the signal electrodes.
The optical modulation system includes a direct modulation which modulates the intensity of light by superimposing a modulation signal on a driving current of a light-emitting element and an external modulation which stores information in the light by providing an optical component for changing the phase, frequency, strength or polarization of the light outside the light-emitting element. In recent years, research and development on an external optical modulator, having an excellent broad-band property and chirping characteristic, has been considerably made, in response to the need for a high-speed modulation and long distance transmission.
2. Description of the Related Art
As the external optical modulator, there are an electro-optical modulator, a magneto-optical modulator, an acousto-optic modulator, an electric field absorption type modulator and the like. The electro-optical modulator uses the electro-optical effect, the magneto-optical modulator uses the magneto-optical effect, the acousto-optic modulator uses the acousto-optic effect, and the electric field absorption type modulator uses the Franz-Keldysh effect and the quantum-confined Stark effect.
One of the examples of the electro-optical modulator will be explained.
In the electro-optical modulator, an optical waveguide, signal electrodes and earthed electrodes are formed on a substrate having the electro-optical effect. The center part of the optical waveguide is branched into two between two Y-branch waveguides to form first and second waveguide arms, so as to structure a Mach-Zehnder interferometer. The signal electrodes are respectively formed on the two waveguide arms, and the earthed electrodes are formed on the substrate in parallel to the signal electrodes with predetermined intervals therebetween. Light is made incident on the electro-optical modulator to propagate through the optical waveguide, branched into two at a first Y-branch waveguide to propagate through the respective waveguide arms, merged into one again at a second Y-branch waveguide, and outputted from the optical waveguide. When electric signals, for example, high-frequency signals are applied to the respective signal electrodes, refractive indexes of the respective waveguide arms change due to the electro-optical effect, and hence the progression speeds of first light and second light, each of which propagates through the first and the second waveguide arms, change. By providing a predetermined phase difference between the electric signals, the first light and the second light are multiplexed at the second Y-branch waveguide with the different phases, whereby the multiplexed light has a mode which is different from that of the incident light, for example, a high-order mode. The multiplexed light with the different mode cannot propagate through the optical waveguide, and hence the intensity of the light is modulated. The Mach-Zehnder type optical modulator (hereinafter abbreviated to the xe2x80x9cMZ optical modulatorxe2x80x9d) realizes the modulation by the process of the electric signalxe2x86x92the change of the refractive indexxe2x86x92the change of the phasexe2x86x92the change of the intensity. The electro-optical modulator like the above is disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei 2-196212.
The electro-optical modulator like the above which controls the phases of the first light and the second light independently by the respective signal electrodes is particularly called as a Dual-Drive optical modulator (hereinafter abbreviated to xe2x80x9cDD optical modulatorxe2x80x9d).
It should be mentioned that the phases of the lights to be multiplexed in the second Y-branch waveguide correspond to the relationship between the phase of the electric signal and the phase of the light at an interaction start point at which the electric signal and the light start the interaction. Hence, in order to obtain the predetermined phase difference between the phase of the first light and the phase of the second light in the second Y-branch waveguide, it is necessary to supply electric signals correlating to the respective signal electrodes, by adjusting the phases of the respective electric signals to the predetermined phases. Conventionally, the phases of the respective electric signals are adjusted by using a phase compensator which is provided outside, because a reference point for the phase adjustment is not provided in the optical modulator.
It should be noted that, in this method of using the phase compensator, there is a disadvantage that the phase compensator needs to be adjusted for each product. Particularly, when the phase is compensated by the cable length, there is a disadvantage that the deviation is caused after the adjustment according to the temperature change, due to the temperature coefficient. Moreover, the adjustment becomes more difficult as the frequency of the electric signal becomes higher, and when a plurality of the electro-optical modulators are used through the cascade connection, it is necessary to adjust the phases of the respective electric signals to be supplied to the respective electric-optical modulators, which makes the adjustment more difficult.
It is an object of the present invention to provide an optical modulator which allows phases of first light and second light to become the predetermined phases at an interaction start point, by matching the phases at points from which respective electric signals are supplied to respective signal electrodes, without using a phase compensator.
The aforementioned object is achieved by an optical modulator comprising a substrate having a predetermined optical effect, an optical waveguide formed on the substrate, being branched into first and second waveguide arms at a first Y-branch waveguide and thereafter merged into one again at a second Y-branch waveguide, a first signal electrode formed on the substrate, for transmitting a first electric signal which interacts with the first light propagating through the first waveguide arm in a predetermined manner, a second signal electrode formed on the substrate, for transmitting a second electric signal which interacts with the second light propagating through the second waveguide arm in a predetermined manner, and an earthed electrode formed on the substrate, wherein, supposing that time for the first electric signal to transmit from a first input end d of the first signal electrode, from which the first electric signal is supplied, to a first interaction start point b, at which the first light and the first electric signal start the interaction, is first progression time t(db), time for the first light to propagate from a branching point a of the first Y-branch waveguide, at which light inputted to the optical waveguide is branched into the first light and the second light, to the first interaction start point b is first propagation time t(ab), time for the second electric signal to transmit from a second input end e of the second signal electrode, from which the second electric signal is supplied, to a second interaction start point c, at which the second light and the second electric signal start the interaction, is second progression time t(ec), and time for the second light to propagate from the branching point a to the second interaction start point c is second propagation time t(ac), the difference between an absolute value of the difference between the first progression time and the first propagation time and an absolute value of the difference between the second progression time and the second propagation time is 0 or the integer multiple of one-fourth of a period T of the first and the second electric signals, which can be expressed as follows:
xe2x80x83|t(db)xe2x88x92t(ab)|xe2x88x92|t(ec)xe2x88x92t(ac)|=0 xe2x80x83xe2x80x83(Expression 1)
or
|t(db)xe2x88x92t(ab)|xe2x88x92|t(ec)xe2x88x92t(ac)|=nT/4 xe2x80x83xe2x80x83(Expression 2) 
wherein n is a positive/negative integer.
Further, this can be also expressed as follows. Supposing that time for the first light to propagate from the first interaction start point b to a merging point k of the second Y-branch waveguide at which the first light and the second light is merged into one is third propagation time t(bk) and time for the second light to propagate from the second interaction start point c to the merging point k is fourth propagation time t(ck), an absolute value of the difference between the sum of the first progression time t(db) and the third propagation time t(bk) and the sum of the second progression time t(ec) and the fourth propagation time t(ck) is 0 or the integer multiple of one-fourth of a period T of the first and the second electric signals, which can be expressed as follows:
|t(db)+t(bk)xe2x88x92(t(ec)+t(ck))|=0 xe2x80x83xe2x80x83(Expression 3) 
or
|t(db)+t(bk)xe2x88x92(t(ec)+t(ck))|=nT/4 xe2x80x83xe2x80x83(Expression 4) 
wherein n is a positive/negative integer.
Further, the first and the second progression times t(db), t(ec) can be adjusted by the length, width, thickness, material, interval between the first or second electrode and the earthed electrodes, of the respective signal electrodes from the respective input ends d, e to the respective interaction start points b, c, or the thickness of a buffer layer between the substrate and the electrode. Namely, the first and the second progression times t(db), t(ec) can be adjusted by the geometric length and by the progression speed of the electric signal.
This kind of optical modulator satisfies the expression 1 or the expression 2, and hence the difference between the phase of the first electric signal at the first input end d and the phase of the second electric signal at the second input end e becomes the difference between the phase of the first electric signal at the first interaction start point b and the phase of the second electric signal at the second interaction start point c. For this reason, the optical modulator allows the first and the second lights, which are branched at the branching point a, to be respectively subjected to the interaction at the first and the second interaction start points b, c, by the difference of the phase of the first electric signal at the first input end d and the phase of the second electric signal at the second input end e.
Therefore, in order to allow the first and the second lights, which are branched at the branching point a and propagating with the same phase, to interact with the first and the second electric signals, respectively, by the predetermined phase difference, all that is needed is to adjust the phase difference between the phase of the first electric signal and the phase of the second electric signal to the predetermined phase difference at the first and the second input ends d, e. Namely, the first and the second input ends d, e are reference positions for adjusting the phases. For this reason, the circuit structure on a periphery of the optical modulator can be simplified since the phase compensator is not necessary.
It should be mentioned that, when the electric signal is an analog signal, its period is a time interval in which the same waveform is repeated, and when the electric signal is a digital signal, its period is a time interval each of which is allocated to one bit.